An amorphous oxide film comprising indium oxide has visible light transmittance, and has a wide range of electric properties ranging from a conductor, a semiconductor to an insulator. Therefore, it has attracted attention as a transparent conductive film or a semiconductor film used in a thin film transistor or the like.
As the method for forming this oxide film, a physical film-forming method such as sputtering, PLD (pulse laser deposition) and deposition or a chemical film-forming method such as the sol-gel method has been studied. A physical film-forming method such as the sputtering method, the PLD method and the electron beam deposition method has been mainly studied since it is a method which can form a film in a large area at relatively low temperatures. When an oxide thin film is formed by a physical film-forming method, in order to form a film uniformly, stably, efficiently and at a high-film forming speed, it is common to use a target formed of an oxide sintered body. In particular, when a target formed of an oxide sintered body is applied to a sputtering method, due to its excellent mass productivity, it can be used for forming a film in a large area such as a flat display.
Regarding oxide sintered bodies, since an n-type semiconductor material containing indium oxide and zinc oxide has been found by Hosokawa, Nakamura or the like, various oxide semiconductors containing indium oxide and zinc oxide, in particular, attracted attention (Patent Document 1). In recent years, a method in which an amorphous oxide semiconductor film prepared by using a target formed of indium oxide, gallium oxide and zinc oxide is driven as a thin film transistor (Patent Document 2) have been studied. However, while an amorphous oxide semiconductor film containing a large amount of zinc oxide has an advantage that it can be etched by an organic acid etching solution (for example, an oxalic acid-based etching solution), it has a problem that it can be dissolved easily in an inorganic acid-based wet etching solution (for example, a mixed acid wet etching solution of phosphoric acid, nitric acid and acetic acid), and hence, wet etching selectivity of Mo (molybdenum), Al (aluminum) or the like is small. Further, an amorphous oxide semiconductor film comprising zinc oxide has a problem that it is etched at a low speed when patterning is conducted by dry etching.
In order to solve these problems, an oxide semiconductor film obtained by adding tin oxide to indium oxide, gallium oxide and zinc oxide and a sputtering target for fabricating the same have been published (Patent Document 3). However, a sputtering target composed of indium oxide, gallium oxide, zinc oxide and tin oxide has many elements to be managed, and hence, the production process or the quality management become complicated.
If a zinc element is contained, when various devices are fabricated on a Si-containing layer such as a silicon substrate, a problem occurs that a zinc element is diffused in the Si-containing layer, whereby the properties are deteriorated. As a result, the applicable device configuration is limited.
An oxide thin film formed of indium oxide, gallium oxide and tin oxide and a target for fabricating the same have been published. However, this is a study aiming at forming a transparent conductive film. Studies on an oxide semiconductor film, in particular, a thin film transistor, have not yet been made. Further, this target contains a large amount of indium, and hence, is not suited to the fabrication of an oxide semiconductor film (Patent Document 3).
In addition, it is disclosed that a compound represented by Ga3−xIn5+xSn2O16 can be synthesized in an area called T phase. However, application of this compound to a target or to the formation of an oxide semiconductor film or the like has not been conducted (Non-Patent Document 1).
In addition, studies have been made on an oxide sintered body target formed of indium oxide, gallium oxide and tin oxide (Patent Documents 4 and 5). However, these studies are intended to fabricate a transparent conductive film. The composition ratio is not adequate to form a semiconductor film, and studies are not made on properties suited to the formation of a semiconductor film.
Further, studies were made on the formation of an InGaSnOx (4≦×≦5) thin film by using an InGaSnO5 target to allow it to serve as an active layer of a thin film transistor (Patent Document 6). However, studies were not made on appropriate target properties, target production methods, composition ratios suited to the process or the like.